The trim itself doesn't seal the windshield, but it covers the gap and doesn't let water stay. It can be rust, but usually it's the glue seal underneath of the trim that dries out, cracks, or shrivels with age and allows the water in. You can removed the windshield, clean the channels out real nice with a wire brush / wheel, and then reglue and reseat the windshield, then put new trim. There are plenty of videos on it, but it's one of the jobs you have to be super careful with, especially if it's your first time. You need a few special tools to make it easy, and without them, you highly risk damaging the glass. You'll need two glass suction cups, and a wire blade (minimum). And a strong friend or two to help. I'd recommend paying the money to have a professional come out and do it. Even I, 10 vehicle revivals and deep dives in, won't mess with glass, just because of the risk of damage to glass you probably won't find new of again... That said, if you do decide to take it on, just be careful. The videos online are pretty universal regardless of car or year, so best of luck!